Shock tubes are a type of fuse cord or blasting cord used in non-electric blast initiation systems. Shock tube was originally described in U.S. Pat. No. 3,590,739 to Persson. Shock tubing typically comprises an elongated, hollow, flexible, small-diameter tube, the inner surface of which is coated with a reactive substance, e.g., a thin layer of detonating or deflagrating explosive composition. Most commonly, this composition consists of a mixture of HMX and aluminum powder. Later shock tube designs such as disclosed in U.S. Pat. No. 4,328,753 to Kristensen encompass multiple plastic layers to provide improved tensile strength and abrasion resistance.
In commercial blasting applications, the shock tubing provides a signal transmission device to transmit a signal to multiple blasting caps in mining or quarrying applications. When initiated, the interior coating of the shock tube transmits a low energy shock wave that travels down the interior of the tube, but without breaching the tube sidewall. A detonator affixed to the end of the tubing is initiated by the shock wave, thereby setting off an attached explosive charge. The U.S. Army has developed shock tube-based initiation systems because of their relative safety. In particular, since the system is non-electric, it is not affected by stray electrical currents so cannot be accidentally initiated by electrical signals. Also, the system does not require special electrical blasting machines as would be required if an electric blasting cap system was used.
In commercial applications, a firing device containing a percussion primer is typically used to initiate the shock tube. For military applications, a self-contained system is desirable. In military systems, an end fitting can be used to position a percussion primer on the end of the shock tube. This type of fitting and initiation system is disclosed in U.S. Pat. No. 6,272,996 B1 to O'Brien et al.
In the field, a spring loaded firing pin device is typically attached to the assembly and used to fire the percussion primer for initiating the shock tube.
More recently, products have been developed for the military with the firing device permanently affixed to the shock tube lead in the factory. This results in a totally self-contained initiation system being delivered in one package to the field. This type of initiation system is disclosed in U.S. Pat. application Ser. No. 10/667,042. As disclosed in this application, the firing devices are mounted on the flange of the spool. The shock tubing is wound around the spool and detonator(s) are crimped to the end of the shock tube.
The length of shock tube on a spool can vary from 80 feet to 1,000+ feet. The length of shock tube allows the field blaster to retreat a safe distance between the charge the detonator is initiating and the firing device that initiates the blast. This system is very robust and useful and has been deployed extensively in military field applications. However the use of a spool (and, of course, box) greatly increases the overall weight and volume of the shock tube package. For covert operations, it is extremely desirable to have a self-contained detonator assembly that is easily carried by a person or one that will fit into a pocket on a vest.